1. Field of the Invention
The present invention is related to an electronic device capable of supplying direct current (DC) power, and more particularly, to an electric device which can supply DC power with low electromagnetic interference (EMI) functions.
2. Description of the Prior Art
As the information technology advances, various kinds of electronic devices are getting popular. Inside each electronic device, numerous types of voltage supplying circuits are utilized to provide the DC (direct current) power for the operating needs of the electronic devices. The most popular DC power supply devices can be classified into two main categories, the linear regulator and the switching power supply. The switching power supply is also called the switching regulator. Compared with the linear regulator, the switching power supply can have better power transfer efficacy, and is widely used in portable electronic devices. Besides that, another tendency is that many more complex and novel functions are being built into portable electronic devices, and the electronic circuit board are designed to be smaller and lighter. The consequences are the density of the electronic circuitry is getting higher, and the electromagnetic interference (EMI) issues among the components upon the electronic circuit board are getting more serious and becoming one of the major design challenges.
About the electromagnetic interference (EMI) come up with a working switching power supply, please refer to FIG. 1, which illustrates a schematic diagram of a switching power supply circuit 10. The switching power supply circuit 10 comprises a power supply 100, a switching controller 102, an upper gate switch M1, a lower gate switch M2, an inductor L1, a capacitor C1 and a load LOAD1. The switching controller 102 controls the electric power transferring from the power supply 100 to the inductor L1 by changing the duty cycle of the switching operation. When the upper gate switch M1 turns on, the inductor current IL1 flowing through the inductor L1 will increase owing to more magnetic energy being stored in the inductor L1. When the inductor current IL1 reaches a predefined upper current bound, the switching controller 102 will immediately turn off the upper gate switch M1, such that the magnitude of the inductor current IL1 can keep falling till the end of the switching cycle. By this way, the power switching circuit 10 can keep the output voltage stable by controlling the on-off actions of the upper gate switch M. Noteworthily, the switch power supply circuit 10 can further turn the lower gate switch M2 on while the upper gate switch M1 is turned off, such that a path for conducting current can be made through loop containing the inductor L1, the load, and the ground GND. Therefore, the continuity of the inductor current IL1 can be maintained. And, FIG. 2 illustrates the loop for conducting current when the lower gate switch M2 is turned on with a dashed arrow sign.
However, when the load LOAD1 of the switching power supply circuit 10 becomes a light load, the duty cycle of the upper gate switch M1 can fall below 10%. Under this condition, for each of the operating cycle, after the upper gate switch M1 turns on for a very short period of time, the lower gate switch M2 will also turn on for a very short of time to prevent the occurrence of the reverse current. However, the fast switching actions of the lower gate switch M2 will result in some negative effect on the operations of the switching power supply circuit 10, such as producing electromagnetic interference (EM1). Please refer to FIG. 3, which illustrates a schematic diagram of the voltage waveform at an endpoint SW of the inductor L1 when the load LOAD1 of the switching power supply circuit 10 becomes a light load. According to the measurements obtained from the experiment, when the load LOAD1 becomes a light load, the upper gate switch M1 and the lower gate switch M2 are expected to stay in the OFF condition for a much longer period of time than it is not a light load. Meanwhile, after the lower gate switch M2 follows the upper gate switch M1 and completes a short OFF->ON->OFF action, the voltage waveform in the endpoint SW of the inductor L1 will produce a large high frequency oscillation, this type of oscillating electromagnetic energy will apply influence on the operations of components neighboring to the switching power supply circuit 10 and the inductor L1, and is considered as an EM1 source. Under certain circumstances, to prevent the operations of the switching power supply circuit 10 to produce the electromagnetic interference (EM1) phenomena and reliability issues becomes very critical, further research efforts are thus required to provide effective solutions to overcome this kind of problem.